This post is a part of automated deployment story

On of the problems with many simple deployment solutions is, the structure of the target environment is embedded into the deployment scripts. This was also the case with the previous, batch based, version of my deployment process. With the advent of PowerShell, everything changed.

Commas

CSV is one of the simplest formats for representing tabular data. It’s easy to read, easy to edit (also in Notepad) and it behaves nicely when stored in VCS. All these properties make it an ideal candidate to be used to describe the deployment environment. Last but not least, PowerShell has very nice built-in features for handling CSV files: Import-Csv, Export-Csv, ConvertTo-Csv and ConvertFrom-Csv. The most interesting one is Import-Csv as it allows to load data stored in file into an object form with one object per row and one property per column. It makes loading entire environment description file as easy as

$machines = Import-Csv environments.txt

The structure of the file is also quite simple. Because I deploy to DEV environment quite frequently (10+ times a day) and that it is done by an automated process, I’ve chosen to store credentials straight in the config file.

IP, Environment, Roles, User, Password
192.168.0.1, DEV, Web|App, devUser, p@ssw0rd
192.168.1.1, TEST, Web,,
192.168.1.2, TEST, App,,

Given these, you can imagine that getting the list of machines in particular environment (you need it to know where to deploy) can be done in another one-liner

$target_machines = $machines | Where-Object { $_.Environment = $target_environment }

Roles

You have probably noticed a column called ‘Roles’ in the description file. A concept of role is key to my deployment strategy. I borrowed the name from cloud computing lingo but the meaning is slightly different. My role is a named set of things that are deployed together and form a logical whole. The current list of roles in my application include web, tools (which is an odd name for application server stuff), tests (this role is installed only on DEV) and db.

As you can see in the sample environment description, a DEV machine serves both roles while in test environment there’s dedicated machine for each role. You can imagine, that production would probably have 3 or 4 machines serving web role and maybe 2 for application role. Having a simple file that has all this information really helps organize things.

Orchestration

Looking from 10.000 ft, the process of deploying one role to one machine is following

  • stop the role
  • update the binaries for this role
  • start the role
  • (optional) verify health of the role

Given that we have multiple machines, each of which potentially serving multiple roles, the question arises how do we orchestrate the whole process so we minimize the down time. The answer, as for all difficult questions, is, it depends. Because it depends, we want to abstract away the concrete orchestration and be able to pick the actual implementation based on requirements at hand. In my project I’ve identified two distinct cases.

If I don’t have to change the structure of transaction database, I can upgrade the machines one by one. To achieve zero downtime I need to fake the response for balancer’s health enquiry telling it that the machine is down before is actually is. This way clients will be served normally before load balancer picks up the faked ‘down’ response and actually removes the machine from the cluster. Then I can safely upgrade this one machine. Last but not least, I have to wait some time before load balancer figures out the machine is up. Voilà! A super-simple zero-downtime deployment.

In case I don’t need to change the structure (and assuming I can’t write code that can work with both structures) I want to upgrade all machines in the environment as quickly as possible. This is where PowerShell v2 jobs come in handy. You can use Start-Job to virtually deploy to each machine in a separate thread of execution. Because the bulk of work is done not on the coordinating machine but on the target machine, this literally cuts the deployment time to the amount it takes to deploy to one machine. Cool, isn’t it?

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